Januzaj in Belgium World Cup squad
The teenager pledged his allegiance to his country of birth last month following reports that he could opt to play for a number of other nations.
Januzaj, who could also have played for Albania, Turkey or Serbia, has been a major positive in an otherwise poor campaign for United and the 19-year-old has been rewarded with a place in Marc Wilmots' World Cup squad.
The tricky winger, who can also operate as a forward, made 27 Premier League appearances for the Old Trafford club and it poised to make his international debut on the biggest stage of all.
Hoffenheim goalkeeper Koen Casteels has been included in Marc Wilmots' squad despite suffering a broken leg in a Bundesliga clash against Hertha Berlin last month.
Wilmots has also included Anderlecht keeper Silvio Proto in his party for the tournament along with Thibaut Courtois and Simon Mignolet, but Proto is only expected to retain his place if Casteels is ruled out.
Lille striker Divock Origi is another teenager who has been given his first senior call-up, with the 19-year-old only having made one appearance at Under-21 level on the international stage.
Origi gets his chance in the absence of Aston Villa striker Christian Benteke, who was ruled out after tearing his Achilles tendon last month.
Wilmots, who guided Belgium to their first major tournament since the 2002 World Cup in Japan and Korea, has named six players on a standby list.
The promising Belgium side will face Algeria, Russia and South Korea in Group H.
Squad in full:
Goalkeepers: Thibaut Courtois (Atletico Madrid), Simon Mignolet (Liverpool), Koen Casteels (Hoffenheim), Silvio Proto (Anderlecht)
Defenders: Laurent Ciman (Standard Liege), Vincent Kompany (Manchester City), Nicolas Lombaerts (Zenit), Daniel Van Buyten (Bayern Munich), Anthony Vanden Borre (Anderlecht), Thomas Vermaelen (Arsenal), Jan Vertonghen (Tottenham)
Midfielders: Kevin De Bruyne (Wolfsburg), Nacer Chadli (Tottenham), Steven Defour (Porto), Moussa Dembele (Tottenham), Marouane Fellaini (Manchester United), Eden Hazard (Chelsea), Adnan Januzaj (Manchester United), Dries Mertens (Napoli), Kevin Mirallas (Everton), Axel Witsel (Zenit),
Forwards: Romelu Lukaku (Chelsea), Divock Origi (Lille)
On standby: Michy Batshuayi (Standard Liege), Guillaume Gillet (Anderlecht), Thorgan Hazard (Zulte Waregem), Radja Nainggolan (Roma), Sebastien Pocognoli (Hannover), Jelle Van Damme (Standard Liege)